The invention relates generally to the field of failure detection circuits for electrical components and more particularly to failure detection circuits for harmonic filters which are used in single sideband transmitters.
Single sideband transmitters usually cover several separated frequency bands with each band requiring a different harmonic filter in order to prevent the transmission of the harmonics of the carrier frequency. The harmonic filter is commonly placed between the final RF (radio frequency) power amplifier stage and the antenna, so that any harmonics generated by this last amplifier will also be filtered out. The frequency response of the harmonic filter is changed (programmed) for each one of the frequency bands that the transmitter will be operated at. If either the harmonic filter or the antenna has the wrong input impedance, large amounts of RF power will be reflected back into the final power amplifier stage. This can cause the expensive power amplifying devices in the final stage to burn out.
Typically, a single directional coupler is placed between the output of the harmonic filter and the antenna. This directional coupler is used to provide an indication of both the forward power delivered to the antenna and the reverse power reflected back by the antenna. The forward power indication is used to control the output of the final amplifier stage so as to maintain the transmitter output at a constant desired level. The reflected power indication is used to shut down the final amplifier stage whenever the ratio of the reflected to forward power indicates a high voltage standing wave ratio (VSWR) for the antenna. This indication of antenna VSWR provides a convenient way of monitoring the antenna input impedance and preventing large amounts of power from being reflected by an antenna impedance mismatch. A disadvantage of this system is that the directional coupler only measures the antenna VSWR and will not shut down the final amplifier when a high VSWR occurs because of either an electrical failure of the filter or the selection of the wrong harmonic filter for a particular frequency band.
A single directional coupler cannot be located prior to the harmonic filter to give a useful measurement of the VSWR of the filter. This is because the forward power into the filter contains the desired signal to be transmitted plus all of its harmonics, whereas the reflected power contains primarily the harmonics. Since the magnitude of these harmonics will vary considerably with both the transmitter operating frequency and the filter input impedance, an accurate indication of the harmonic filter VSWR cannot be obtained. Also, this single directional coupler would measure the combined VSWR of both the harmonic filter and the antenna, and therefore be incapable of distinguishing a filter failure from an antenna failure. In addition, this single directional coupler could not be used to accurately monitor and control the actual transmitted output power, since the monitored forward power includes the signal to be transmitted and all of its harmonics and none of these harmonics will be actually transmitted. Thus the prior art does not provide an adequate system of protecting against the failure or misprogramming of a harmonic filter in a single sideband radio which operates over several discrete frequency bands.